Superspreading driven by Marangoni flow
Introduction
The phenomena of wetting and spreading of a water droplet on solid hydrophobic surfaces is frequently encountered in many industrial applications, such as coatings, printing and painting. Surfactants play an important role in enhancing a liquid's ability to wet and spread on solid surfaces. Surfactants are used as emulsifiers and dispersants in pesticide formulations to assist with the delivery of agrochemicals to a spray mixture. A non-ionic surfactant is often added to a tank mix containing a herbicide for the purpose of improving the spreading and wetting of the spray deposit on leaf surfaces [1], [2]. Generally, the epicuticular wax on a leaf acts as a substantial barrier to wetting. Water alone tends to bead-up and roll off the leaf, which can make spray applications ineffective. Because surfactants have the ability to reduce the surface tension of water and to induce a surface tension gradient, they enable spray solutions to more effectively wet waxy leaf surfaces, thereby increasing the amount of spray retained on the leaf. As a result, surfactants can make spray applications more reliable by improving the delivery of agrochemicals.
One relatively new class of surfactant, the trisiloxane alkoxylate ‘superspreaders’ has the unique ability to spread spray solutions across difficult-to-wet leaf surfaces to a greater degree than conventional surfactants. Among the known surfactants, the trisiloxane ethoxylate with 8 ethoxy groups (TS8EO) has been found to be one of the best water-spreading agents for hydrophobic surfaces [3], [4]. The overall spread area achieved by an aqueous droplet containing a trisiloxane surfactant can be as much as 50 times greater than water, and 25 times more effective than a conventional surfactant [5]. This increase in spreadability can provide an improvement in spray coverage on target plants, and even allow growers to use less water to achieve the same or a better degree of coverage as compared to conventional treatments [6], [7].
Fig. 1 illustrates this point by comparing the relative spreading properties of water alone, with 0.25 wt.% Triton® X-100 (polyoxyethylene (10)-octylphenyl ether, from Union Carbide Corp., USA) or with an aqueous solution of 0.1 wt.% trisiloxane ethoxylate, Silwet L-77® surfactant (TS8EO, methyl capped, from OSi Specialties, Crompton Corp., USA) on velvetleaf (Abutilion theophrasti). Water alone on velvetleaf gives a high contact angle (>90°), and has a relatively poor spreading ability [8]. Likewise, the droplet containing Triton X-100 gives a slight reduction in the contact angle due to the decrease in the aqueous surface tension, and a corresponding increase in the spread area on the leaf. The droplet containing Silwet L-77 surfactant spreads spontaneously, giving a significant increase in the spread area, and a rapid reduction in the contact angle.
Although there is an increased interest in the various applications for superspreaders, the mechanisms for this phenomenon are not well understood [4]. A number of explanations have been proposed for this superspreading phenomenon, mostly related to the configuration of the trisiloxane molecules at the interfaces [3], [9], [10], or the formation of a precursor film on a solid surface [10]. It has also been suggested that the formation of vesicle type aggregates is a requirement for trisiloxane mediated superspreading [4], [10], [22]. It is true that TS8EO ‘solutions’ are turbid above approximately 0.01% concentration due to the presence of the aggregates and vesicles formed by the surfactant molecules. Some have speculated that surfactant vesicles and/or other aggregates disintegrate and provide an efficient delivery of surfactant molecules to the contact surfaces (or the spreading front) and enhance spreading [10]. However, as we shall see, the presence of vesicles or aggregates is not a requirement, nor does it explain the superspreading effect observed with trisiloxane alkoxylate solutions.
In our opinion, the main questions on superspreading are:
- 1
What is the driving force that promotes the rapid superspreading of trisiloxane surfactant solutions over a large area, even on strongly hydrophobic surfaces?
- 2
What is special about trisiloxane ethoxylate surfactants that makes them better spreaders than conventional organic surfactants?
In this article, we will use some simple experiments and physical concepts to support our proposed mechanism for the superspreading phenomenon, as facilitated by the Marangoni effect. We will discuss the role of surfactant aggregates on the spreading process, as well as consider the molecular conformation of the trisiloxane ethoxylate at the gas/liquid interface, in relation to the packing of the adsorption layer, and the surface tension gradient.
The classical physical concepts for the spreading of liquids on a solid surface can be traced back to two basic mechanisms. The first mechanism is influenced by a disparity in the surface or interfacial tensions at the three-phase contact line. If the gas/solid surface tension (σg/s) is greater than the sum of the liquid/gas and liquid/solid interfacial tensions (σl/g+σl/s), the liquid will spread on the solid surface until the surface tension balance is restored. According to this concept, spreading is driven by wetting (i.e. solely by the unbalanced capillary force on the three-phase contact line) and the three-phase contact angle decreases with time as the droplet spreads. When the liquid/gas surface tension is decreased, and the other two tensions are constant, the droplet will spread until the surface tensions (balanced at the three-phase contact line) are restored. The surface tension balance is restored usually after several minutes because surfactant diffusion and adsorption is time dependent. In general, the lower the surface tension, the faster the droplet spreads. For example, solutions of a typical commercial surfactant, such as Triton X-100, have a spreading rate of several centimeters per minute, and the drop expands its area approximately 100%.
Another mechanism of droplet spreading is due to the Marangoni effect [11], [12], [13], [14], [15], [16].
It has long been known that the spatial variation in surface tension at a liquid/gas surface results in added tangential stresses at the surface; this results in a surface traction that acts on the adjoining fluid, giving rise to the fluid motion in the underlying bulk liquid. The tangential force per unit area is given by:where ∇s denotes the surface tension gradient. A positive value for ∇sσ indicates that the liquid tends to move in a direction from a low to a high surface tension. The liquid droplet spontaneously moves over the solid surface without the application of external force.
If the liquid/air surface tension is greater at the front surface of the spreading drop than in the center of the drop, then the difference in surface tension establishes a gradient and will create a Marangoni flow. The Marangoni flow will push the drop outward and cause it to spread over a large area. It is also possible for the two mechanisms to operate, but at different time scales. For instance, at the beginning (early-times) when the droplet is placed on the solid, due to high stretching, the droplet spreads under Marangoni flow. With time, when the gradient is diminished due to surfactant adsorption, the spreading is governed by wetting forces (i.e. due to the unbalanced tensions in the three-phase contact region).
Section snippets
Materials
Trisiloxane ethoxylate, Silwet L-77® commercial surfactant (TS8EO, methyl capped, from OSi Specialties, Crompton Corp., USA was used to study droplet spreading on a polystyrene surface (Petri dish). The polystyrene surface was pre-washed with water purified with Milli-Q water and dried at room temperature (humidified 30–40%). The surfactant solutions were prepared with water purified by Milli-Q water system. The experiments were carried out with a freshly prepared, well-homogenized surfactant
Results and discussion
Several experiments were conducted in order to identify the mechanisms governing the superspreading of trisiloxane ethoxylates on hydrophobic surfaces and the role of surfactant aggregates in superspreading.
In the first experiment, the early-time rate of spreading for 0.1 ml drops of 0.1 wt.% Silwet L-77 surfactant (TS8EO) on polystyrene (Petri dish was determined using the optical arrangement shown in Fig. 2. The Petri dish was placed in a glass cell under controlled humidity and the drop
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2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :Here R(t) denotes the radius of the expanding liquid film. The superspreading mechanism is generally attributed to the Marangoni effect (flow generated by the surface tension gradient) [34]. The Marangoni flow is driven by the concentration gradient of surfactant, which is caused by non-uniform surfactant adsorption [35–37].